Field of the Invention
The present invention relates to an object information acquiring apparatus that acquires information on the inside of an object.
Description of the Related Art
Attempts have been made to noninvasively acquire information on the inside of a living body using light. For example, when a living body that is an object is irradiated with measurement light such as pulsed laser light, an acoustic wave is generated when the measurement light is absorbed by the biological tissue in the object. Information on the inside of the living body can be acquired by receiving and analyzing the acoustic wave (typically an ultrasound wave). Such a technique is referred to as photoacoustic imaging.
The photoacoustic imaging implements imaging of information related to an absorption coefficient with respect to the inside of the object. The absorption coefficient is the rate at which the biological tissue absorbs light energy. Measuring the absorption coefficient allows acquisition of the concentrations of components of the biological tissue. In particular, the use of light with a wavelength likely to be absorbed by hemoglobin in the blood enables the concentration ratio of oxyhemoglobin to deoxyhemoglobin to be determined. This allows the oxygen saturation of the biological tissue to be calculated. As is known, if a tumor tissue is present in the living body, the oxygen saturation decreases in the corresponding site. Thus, diagnosis for tumor is expected to be enabled by measuring the absorption coefficient.
Now, a method for calculating the absorption coefficient with respect to the inside of the living body based on the received acoustic wave will be described. First, the received acoustic wave is reconstructed to generate a distribution of initial sound pressure of a sound source. The initial sound pressure can be expressed by multiplying the intensity of light having reached a target area, the absorption coefficient of the light, and a Grueneisen constant together. That is, the distribution of the absorption coefficient can be obtained by dividing the distribution of initial sound pressure by the Grueneisen constant and by the distribution of the light intensity.
When the object is a living body, the distribution of light intensity needs to be determined in order to obtain the absorption coefficient because the Grueneisen constant is considered to be a known predetermined value. The distribution of light intensity can be calculated based on the optical characteristics of the biological tissue. The biological tissue has two optical characteristics: a light absorption characteristic (hereinafter referred to as a background absorption coefficient) and a light scattering characteristic (background scattering coefficient) for an area through which the light passes after being provided to the object and before reaching the light absorber. The two coefficients are collectively referred to as a background optical coefficient. The background optical coefficient significantly affects the calculation of the absorption coefficient and thus needs to have an accurate value.
The background optical coefficient can be measured by irradiating the object with measurement light and detecting light having propagated through the object. For example, Japanese Patent Application Laid-open No. 2002-139420 and Non-Patent Document 1 describe apparatuses that measure the background optical coefficient using a time resolved measurement method based on pulsed light. Furthermore, Japanese Patent Application Laid-open No. H07-159239 describes an apparatus that measures the background optical coefficient using a phase modulation measurement method based on intensity modulated light.    Non-Patent Literature 1: “Quantitative measurement of optical parameters in normal breasts using time-resolved spectroscopy: in vivo results of 30 Japanese women”, Kazunori Suzuki M.D.; Yutaka Yamashita; Kazuyoshi Ohta; Masao Kaneko; Masayuki Yoshida M.D.; Britton Chance, Journal of Biomedical Optics 1(03), pp. 330-334